Neutron spectrometer, real-time dosimeter and methodology using three-dimensional optical memory

ABSTRACT

A dosimetry method, dosimeter and system characterized by the steps of storing information in a three dimensional optical memory element, then exposing the optical memory element to neutron or other high LET radiation to alter the information stored in the optical memory element as a function of the radiation to which the optical memory element is exposed, and then retrieving the altered information from the optical memory element for subsequent analysis. The altered information is used to provide a measure of both the radiation dose and energy. In a preferred embodiment, the optical memory device is a 3-D ORAM comprising a volume of a transparent polymer doped with a light sensitive chemical and, in particular, spirobenzopyran. Also disclosed are a spectrometer for monitoring neutron and other types of radiation, an electronic dosimeter for providing real time monitoring of radiation exposure, and associated methodologies, all based on use of an optical memory element.

RELATED APPLICATION DATA

This application is a continuation-in-part of application Ser. No.07/974,207, filed on Nov. 10, 1992, now U.S. Pat. No. 5,319,210, issuedJun. 7, 1994, which application is hereby incorporated herein byreference.

FIELD OF INVENTION

The invention herein described relates generally to radiation dosimetryand, more particularly, to a neutron dosimeter and associated dosimetrymethod that allows precise neutron dose measurements. The invention alsohas application to other types of high linear energy transfer (LET)radiation such as protons, heavy ions, etc. and to microdosimetry, andvarious apparatus are disclosed.

BACKGROUND

The fraction of radiation dose from neutrons received by radiationworkers is increasing as a result of growth in the nuclear powerindustry, the development of nuclear reactor technology, and thepotential use of neutrons for radiotherapy. Unfortunately, neutrondosimetry has been a difficult problem due to low neutron sensitivityand energy dependence of existing dosimetry methods. Prior art neutrondosimetry methods include thermoluminescence dosimetry (TLD), solidtrack detector methods using, for example, electrochemically etchedCR-39 foil or NTA film, and fluid track detector methods using, forexample, superheated bubble detectors (SSD).

The foregoing methods may not have the energy response and sensitivitynecessary to meet the more exacting needs of neutron dosimetry. TLDsuffers from high energy dependence, which may result in an error of asmuch as a factor of ten or more if the neutron energy spectrum is notknown. NTA films have response functions that may cause even greatererrors for many operational situations. Major unaddressed problems withCR-39 are the lack of sensitivity at low neutron energies, energydependence and poor sensitivity at high energies. The more recentlydeveloped superheated drop detector has been shown to suffer fromserious drawbacks including a fourfold reduction in the energy responseat energies from 0.144 MeV to 5 MeV.

Other neutron dosimetry methods that have been proposed rely onelectrical property changes, such as soft errors which arise in dynamicrandom access memories (DRAMs) through interaction with chargedparticles, particularly, alpha particles. For use as a neutrondosimeter, a converter is used to interact with the neutrons andgenerate protons or alpha particles. Accordingly, the overallperformance of the neutron dosimeter is at least in part dependent onthe performance of the converter which may be a foil layer applied tothe DRAM. Moreover, a neutron/alpha converter has the disadvantage ofincreasing the dosimeter size and complicating the dose readinginterpretation. In addition to the need for a converter, the material ofthe DRAMs is not tissue equivalent and there still remains the problemof energy dependence.

Neutron dosimetry is recognized as being a difficult problem in healthphysics. Recently there has been a reevaluation of the biologicalhazards associated with neutron exposure and, consequently, there is anurgent need for a neutron dosimetry method that provides precise neutrondose measurement over a wide range of neutron energies. Moreparticularly, a need exists for a neutron dosimeter and dosimetry methodthat solves the two major unsolved problems of neutron dosimetry: (1)inability to measure the neutron energy which results in errors inestimating the dose equivalent and (2) poor sensitivity at high and lowneutron energies. There is needed a neutron dosimeter that is sensitiveat both high and low energies and is capable of characterizing theexposure energy spectrum, thereby to permit accurate neutron dosemeasurements.

SUMMARY OF THE INVENTION

The present invention satisfies the aforesaid need by taking an entirelynew approach to neutron, and more generally high LET radiation,dosimetry that is characterized by the use of an optical memory devicecomposed of a volume of material containing a photoactive substancewherein an energy induced three-dimensional inhomogeneity pattern may beproduced and/or detected optically as by use of directed electromagneticradiation. More particularly, the invention uses a two photon based,three-dimensional optical random access memory (3-D ORAM) thatheretofore has been proposed as a memory device for computers whereinmassive quantities, such as many gigabytes, of data is to be stored. The3-D ORAM is a volume, typically a cube, of transparent polymer dopedwith a light sensitive chemical that can be written and read using twolaser beams that simultaneously strike the material to alter at theirintersection an optical characteristic of the material.

According to the invention, a dosimetry method comprises the steps ofstoring information in a three dimensional optical memory element, thenexposing the optical memory element to neutron or other high LETradiation to alter the information stored in the optical memory elementas a function of the radiation to which the optical memory element isexposed, and then retrieving the altered information from the opticalmemory element for subsequent analysis. More particularly, certaininformation is written on the optical memory element, which informationbecomes altered by exposure to the radiation. The altered information islater read from the optical memory and analyzed to provide a measure ofboth the radiation dose and energy.

In a preferred embodiment, the optical memory device is a 3-D ORAMcomprising a volume of a transparent polymer doped with a lightsensitive chemical and, in particular, spirobenzopyran. Binaryinformation is stored in the ORAM by excitation of the photochronicchemical dopant molecule to a higher energy state using laser light.When the molecule absorbs simultaneously two photons, an opticalcharacteristic thereof, such as color, changes and records a bit.

When neutron radiation interacts with the hydrogen and carbon composingthe ORAM material, it will create energetic heavy ions. Those heavy ionswill cause a local energy deposition and the resultant localizedtemperature increase will cause the excited or "written" molecule ormolecules in the vicinity to revert to their lower energy or "unwrittenstate". In computer memory terminology, the interaction with neutronradiation will cause an error or errors to occur in the form of a bitflip or flips, i.e., a change from a written binary "1" state to anunwritten "0" state. The number of "errors" or "bit flips" will berelated to the neutron dose, and therefore the dose can be calculated.The local energy deposition will also occur from interaction with othertypes of high LET radiation such as proton and heavy ion radiation.

In addition, the energy of the absorbed radiation may be determined fromthe spatial distribution of the bit flips in that neutrons (or otherhigh LET particles) of different energies will produce different trackstructures in the ORAM. An energy measurement can be made by relatingthe radiation energy spectrum to the track structure produced by theinteractions with the hydrogen and carbon atoms composing the ORAM. Oncethe energy is known, energy dependent quality factors can be applied toprovide dose equivalent.

The present invention enables a sensitivity many orders of magnitudegreater than the sensitivity afforded by existing neutron dosimetrymethods at both high and low energies. This arises from the high storageand interaction density of ORAM which is 10¹² bits/cm³. Furthermore,conventional ORAMs useful in practicing the invention are composed ofhydrogen and carbon atoms. This provides tissue equivalence while at thesame time eliminating the need for, and the drawbacks associated with,an external alpha or proton radiator that heretofore was employed inneutron dosimetry. The present invention also is useful in practicingmicrodosimetry.

According to another aspect of the invention, there is provided a highLET dosimeter comprising a dosimeter element and a holder for saiddosimeter element. The dosimeter element is formed by an optical memorydevice and the holder includes means whereby the holder may be worn by aperson whose radiation dose exposure is to be monitored. In a preferredembodiment, the dosimeter element is removable from the holder so thatit may be "read" in a reader intended for this purpose.

According to yet another aspect of the invention, there is provided adosimetry reader comprising means for removably receiving an opticalmemory element that has been exposed to high LET radiation, means forretrieving information from the optical memory element, and means foranalyzing the information retrieved from the optical memory element toprovide a measure of radiation dose and/or neutron energy.

According to a further aspect of the invention, there is provided adosimeter system comprising a dosimeter including an optical memorydevice for exposure to radiation, a reader for retrieving informationfrom the optical memory device after exposure to radiation, and meansfor analyzing the retrieved information to provide a measure ofradiation dose and/or energy.

According to still another aspect of the invention, there is provided amethod for monitoring ionizing radiation exposure comprising the stepsof storing information in a three dimensional optical memory element,then exposing the optical memory element to ionizing radiation to alterthe information stored in the optical memory element as a function ofthe radiation to which the optical memory element is exposed, retrievingthe altered information from the optical memory element, and analyzingthe altered information retrieved from the optical memory element toextract radiation exposure information therefrom.

According to yet another aspect of the invention, there is provided amethod of performing microdosimetry comprising the steps of storinginformation in a three dimensional optical memory element, then exposingthe optical memory element to radiation to alter the information storedat a plurality of memory locations in the optical memory element throughlocal interaction with the radiation to which the optical memory elementis exposed, and retrieving the altered information from the opticalmemory element by reading the memory locations to determine the locationof the memory locations that have been altered through local interactionwith the radiation thereby to obtain a measure of the spatialdistribution of radiation dose within the optical memory element.

The invention herein described also provides a spectrometer formonitoring neutron and other types of radiation, an electronic dosimeterfor providing real time monitoring of radiation exposure, and associatedmethodologies, all based on an optical memory unit sensitive to theradiation being monitored. Preferred embodiments of the spectrometer anddosimeter are characterized a three dimensional optical memory elementhaving a plurality of memory locations that may be written from a firstenergy state to a second energy state, and which memory locations becomealtered by reversion from their second energy state to their firstenergy state through localized interactions between incident radiationand molecules composing the optical memory element; means for readingthe optical memory element to retrieve therefrom information alteredthrough interaction with incident radiation; means for analyzing thealtered information retrieved from the optical memory element to extractradiation dose information therefrom; and means for displaying the doseinformation extracted from the optical memory element. The means foranalyzing preferably includes a neural network computer apparatus fordetermining the radiation energy as a function of the spatialdistribution of the memory locations that have reverted to their firstenergy state.

The invention also provides a high LET radiation dosimetry methodcomprising the steps of storing information in a three dimensionaloptical memory element having a plurality of memory locations byexciting the memory locations from a first energy state to a secondenergy state; exposing the optical memory element to high LET radiationto alter the information stored in the optical memory element as afunction of the radiation to which the optical memory element isexposed, the excited memory locations reverting from their second energystate to their first energy state through localized interactions betweenthe radiation and molecules composing the optical memory element;retrieving the altered information from the optical memory element forsubsequent analysis by reading the memory locations to determine thespatial distribution of the memory locations that have reverted to theirfirst energy state; analyzing the altered information retrieved from theoptical memory element to extract radiation dose information therefrom,said analyzing step including using a neural network computer apparatusfor determining the radiation energy as a function of the spatialdistribution of the memory locations that have reverted to their firstenergy state. Usually, the analyzing step further includes determiningthe radiation dose as a function of the number of the memory locationsthat have reverted to their first energy level.

The foregoing and other features are hereinafter described andparticularly pointed out in the claims, the following description andthe annexed drawings setting forth in detail illustrative embodiments ofthe invention, these being indicative, however, of but a few of thevarious ways in which the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (from Parthenopoulos and Rentzepis, Science, Vol. 245, 843(1989)) is an energy level diagram of the "write" and "read" forms ofthe spirobenzopyran molecule in a polymer matrix.

FIG. 2 (from Stein, Byte, March 1992, 168) is diagram showing a 3-D ORAMbased on a two-photon process using two orthogonal laser beams.

FIG. 3 is a diagrammatic illustration of a neutron dosimetry systemaccording to the present invention.

FIG. 4 is a diagrammatic illustration of a neutron dosimeter accordingto the invention.

FIGS. 5 and 6 (from Bolch et al., Health Physics, Vol. 53, 241, 245-246(1987)) are illustrations respectively showing simulated trackstructures produced by a 500 keV recoil proton and an 80 keV recoil Cion in gas.

FIG. 7 is a representation of a neural network useful in carrying outthe present invention.

FIG. 8 is a diagrammatic illustration of a neutron spectrometeraccording to the present invention.

FIG. 9 is a diagrammatic illustration of a real time electronicdosimeter according to the present invention.

DETAILED DESCRIPTION

The neutron dosimetry method of the invention uses as a dosimeter anoptical memory element composed of a volume of material containing aphotoactive substance wherein an energy induced three-dimensionalinhomogeneity pattern may be produced and/or detected optically as byuse of directed electromagnetic radiation. The method generallycomprises the steps of storing information in a three dimensionaloptical memory element, then exposing the optical memory element toneutron radiation to alter the information stored in the optical memoryelement as a function of neutron radiation to which the optical memoryelement is exposed, and then retrieving the altered information from theoptical memory element for subsequent analysis. The steps are more fullydescribed below as are details of a preferred optical memory element,devices and system useful in practicing the method.

A preferred optical memory element useful as a neutron dosimeter is athree dimensional optical random access memory (3-D ORAM) of the typepreviously proposed for use as a memory storage device in computers. Aknown 3-D ORAM is composed of a transparent polymer doped with a lightsensitive chemical called spirobenzopyran. The spirobenzopyran moleculesembedded in the polymer matrix have two isometric forms that change fromone to the other in response to energy level changes stimulated byelectromagnetic energy. 3-D ORAM elements composed of spirobenzopyranembedded in a polymer matrix may be obtained from the University ofCalifornia, Irvine, Calif., where the 3-D ORAM was developed as reportedin Parthenopoulos and Rentzepis, "Three-Dimensional Optical StorageMemory", Science, Vol. 245, 843-845 (1989). The spirobenzopyran moleculeand its major properties are described in Bertelson, Techniques ofChemistry: Photochromism, Vol. 3, Wiley-Interscience, New York, 1971, p.45.

The storage and retrieval of meaningful information in and from the ORAMmay be effected by using a Nd:YAG solid state laser system to write andread information in a binary format provided by the two distinct formsof spirobenzopyran. The Nd:YAG laser produces both 532 nm and 1064 nmlaser beams that are particularly useful in writing and reading the ORAMusing two-photon absorption. Two-photon absorption is the excitation ofa molecule to an electronic state of higher energy by the simultaneousabsorption of two photons, and the physics thereof is further describedin R. M. Macfarlane, J. Lumin, 38, 20 (1987).

When the spirobenzopyran molecule absorbs simultaneously two 532 nmphotons, it changes color and records a "bit". As illustrated in FIG. 1,the first photon provides excitation to an intermediate virtual stateand the second photon further excites the molecule to a stable excitedstate. Since the virtual state is unstable, both photons must overlap inboth space and time for a transition to occur. In binary logic, the format the left may represent a "1" and the form at the right may representa "0".

The read operation also relies on the simultaneous absorption of twophotons, but the two photons are of longer wavelengths. Two 1064 nmphotons can be used to simultaneously excite the spirobenzopyranmolecules. Only the molecules that have been written will absorb thelonger wavelengths and emit fluorescence that may be detected using asensitive optical detector array.

Information may be written and read at selected locations within theORAM by using two mutually orthogonal laser beams focussed tosimultaneously excite an "addressed" molecule as depicted in FIG. 2. Forwriting a bit, two 532 nm beams are focussed at the addressed locationwithin the volume of the ORAM thereby exciting the molecule to itshigher energy state. For reading a bit, two 1064 nm beams are focussedat the addressed location and fluorescence is looked for to see if theaddressed bit had been written. Erasing may be achieved by either lightfor erasing selected bits or by temperature for bulk erasure. As will beappreciated from the following description, erasure is not necessary asnormally, for reuse, the entire ORAM is written to rejuvenate the ORAMfor reuse as a neutron detector. Beam splitter and focussing optics maybe used to provide the two laser beams that simultaneously strike theORAM and write or read a bit at their intersection. For writing, it mayprove to be preferable to use a weak 532 nm beam and a strong 1064 nmbeam to avoid a potential complication arising from two-photonabsorption being induced by each beam separately when two beams of equalwavelength are used.

For details of a system for writing and reading ORAM, reference may behad to Hunter et al., "Potentials of Two-Photon Based 3-D OpticalMemories for High Performance Computing", Applied Optics, Vol. 29, No.14, 10 May 1990, which is hereby incorporated herein by reference. Thepresent invention may be practiced using any suitable hardware andassociated software for implementing the herein described write and readfunctions. Moreover, the ORAM write/read equipment may have integratedtherein or may be interfaced to a computer, such as a conventionalprogrammable microcomputer, that performs the hereinafter discussed dataanalysis and control functions. FIG. 3 shows a representative system 10including an ORAM dosimeter reader 12 and computer 14.

According to the invention, an optical memory element, such as the abovedescribed ORAM, is used as the neutron sensitive element of a dosimeter18. As diagrammatically depicted in FIG. 4, the ORAM 20 will typicallybe housed in a holder 22 for protection and to facilitate mounting at amonitoring site or wearing of the dosimeter on a person such as byclipping the holder to the user's clothes or by direct attachment to theuser's body. The holder may be multi-component holder including, forexample, a case 24 for the ORAM dosimeter element and a carrier 26 forthe case. The case 24 should be opaque to light to avoid the possibilityof non-neutron induced response in the ORAM dosimeter element. Dependingon the nature of the hardware selected to write and read the dosimeter,the ORAM dosimeter element may need to be removable from the case and/orholder for placement in the write/read equipment. The carrier mayinclude a clip or other attachment/wearing device.

The ORAM dosimeter element 20 is prepared for use by writing a bit ateach memory location to be used for neutron dose monitoring. Preferably,all addressable memory locations are written to maximize the sensitivityof the ORAM. Once written the dosimeter element may be placed in theholder 22 and distributed or placed for monitoring exposure to neutrons.For example, the dosimeter may be worn by a person whose exposure toneutrons is to be monitored. The ORAM dosimeter will then be exposed tothe same radiation as the person. After a prescribed period of time, thedosimeter is retrieved so that it may be read to extract therefromradiation exposure information.

When neutron radiation interacts with the hydrogen and carbon composingthe ORAM material, it will create energetic heavy ions. These heavy ionswill cause an error to appear in the ORAM by local energy deposition inthe vicinity of the exited molecule. The resulting temperature increaseerases the written form of the molecule causing an error to occur in theform of a bit flip (a change of "1" to "0"). The measurable number oferrors will be a function of and in principle proportional to theneutron dose. Accordingly, the absorbed neutron dose can be calculatedas a function of the number of measurable errors. Absorbed dose is theprimary physical quantity used in radiation dosimetry (often referred tosimply as dose). Absorbed dose is defined as the energy absorbed perunit mass from any kind of ionizing radiation in any kind of matter.

The neutron energy can be calculated from the structure of the iontracks produced within the ORAM. By way of analogy, different recoilparticles produce different track shapes in gas ionization chambers.FIGS. 5 and 6 show a significant difference in simulated track structureproduced by a 500 keV recoil proton as compared to an 80 kev recoil Cion in gas. The distribution of the bit flips in the ORAM, and moreparticularly the bit-flip density distribution along the recoil chargedparticle tracks within the ORAM, will reveal the track structuredistribution of the secondary charged particles produced by neutroninteractions with the hydrogen and carbon atoms composing the ORAM.Accordingly, the functional relationship between the neutron energy andthe spatial distribution of bit flips in the ORAM may be used to provideenergy measurement. Once the energy is known, applying energy dependentquality factors provides the required dose equivalent. The quantity doseequivalent is used to allow for different biological effectiveness ofdifferent kinds of radiation for radiation protection purposes. The doseequivalent H is defined as the product of the absorbed dose D and adimensionless factor Q, which depends on the type of radiation. Forgamma rays, X-rays, electrons and positrons, the value of the qualityfactor is 1. For neutrons and heavy charged particles the quality factoris in the range of 2-20 depending of the energy and type of particle.

Algorithm development for a given ORAM dosimeter element may beapproached in a manner similar to the method described in Bolch et al.,"A Method of Obtaining Neutron Dose and Dose Equivalent From DigitalMeasurements and Analysis of Recoil-Particle Tracks", Health Physics,53, 241-253 (1987), which is hereby incorporated herein by reference. Asuitable algorithm may calculate the track length L of the recoil eventby considering the two cells most likely to contain the true endpointsof the track, and the average diameter R of the track by performingthree-dimensional scanning for bit-flip events in the neighborhood ofthe track axis. R and L may then be used to unfold neutron doseequivalent using predetermined relationships.

As will be appreciated by the reader, an improved neutron sensitivity isachieved by several orders of magnitude taking advantage of the highstorage density of ORAM on the order of 10¹² bits/cm³ as compared to thestorage density of 10⁸ bits/cm³ for a conventional silicon based DRAMsuggested in the literature for thermal neutron dosimetry. Moreover,ORAM, being composed of hydrogen and carbon atoms required for neutroninteractions, provides tissue equivalence while eliminating the need foran external alpha or proton radiator often employed in neutrondosimetry. Alpha or proton radiators have the disadvantage of increasingthe dosimeter size and complicating the dose reading interpretation.

Other Applications

Although the invention as thus far described has been in relation toneutron dosimetry, the principles of the invention can also be appliedto other types of radiation and, in general, high linear energy transfer(LET) radiation such as protons, heavy ions, etc., and also tomicrodosimetry. As above discussed, neutron radiation interacts with thehydrogen and carbon of the ORAM material and creates energetic heavyions which cause local energy deposition. This local energy depositionwill also occur from interaction with other types of high LET radiationsuch as proton and heavy ion radiation.

In fact, the above described neutron dosimeter may require a filter toblock high LET particles other than neutrons where their presence is ofsignificance. For most dosimetric applications on earth this willnormally not be a problem because protons and heavy ions typically willnot be present in any significant quantity in most instances. Thesituation is different in space applications where high energy protonsconstitute the predominant form of high LET radiation.

Accordingly, the above described methodology, dosimeter and equipmentcan be used to perform high LET radiation dosimetry including, inparticular, proton dosimetry. The only difference is that the incidentradiation directly interacts with the ORAM material to effect localizedheating rather than through creation of secondary particles that causelocalized heating as in the case of neutrons. In either case, theresulting temperature increase erases the written form of the excitedmolecule causing a error to occur in the form of a bit flip as abovedescribed. Accordingly, the absorbed dose can be calculated as afunction of the number of measurable errors. Also, the high LETradiation energy can be calculated from the structure of the high LETparticle tracks produced within the ORAM. Moreover, the nature orspecific form of these tracks may possibly be used to discriminatebetween different types of high LET radiation such as between protonsand neutrons.

The invention can also be used to practice microdosimetry and, in doingso, overcomes a fundamental problem that plagued earlier microdosimetrytechniques. Microdosimetry requires measurement of the microscopicdistribution of dose in an irradiated solid body. Knowledge of how abody absorbs radiation locally is important to research on thebiological effects of absorbed radiation. A problem in the past was thatmeasurements couldn't be made inside the solid body. Because of this,researchers would use computer simulations or look at local effects in agas and then extrapolate them to a solid body.

The present invention enables measurement of the distribution of doseinside a solid and, in particular, a solid that is tissue equivalent.The radiation induced "errors" can be read and mapped to provide athree-dimensional picture of the radiation distribution inside the ORAM.

Energy Determination Using A Neural Network

Neural networks are information processors inspired by the biologicalnervous system. They are computer-based simulation of living neurons (anerve cell with all of its processes), which work fundamentallydifferent than conventional computing. A neural network has thecapability to learn from its own experience. Neural networks have beenproven to be particularly useful for pattern recognition applications.Boone J. M., Sigillito V. G., and Shaber S. G., "Neural networks inradiology: An introduction and evaluation in signal detection task",Med. Phys. 17, 234-241 (1990). This feature makes the networkparticularly useful for determining the energy spectra based on theshape (microdosimetric track structure) of the particle tracks in theoptical memory element. A typical neural network is designated generallyby reference numeral 30 in FIG. 7. The basic building block is a node(artificial neuron or processing element), represented by the circles.

As illustrated in FIG. 7, the network typically consists of aninput-layer of processing elements 32, an output-layer of processingelements 33, and one or more hidden-layers of processing elements 34 and35. Each link between the processing elements, shown as straight lines,carries a particular weight. The intelligence of the network resides inthe values of these weights. In an asynchronous fashion, each processingelement computes the sum of products of the weight of each input linemultiplied by the signal level on that input line. If the sum ofproducts exceeds a preset activation threshold, the output of theprocessing element typically is computed using a nonlinear function(sigmoid, for example). Learning is achieved through adjustment of thevalues of the weights. The value of weights are determined by presentingthe network with "training material" in the form of a variety ofinput/output data pairs. During successive iterations through thetraining set, the weights are being continuously updated by a learningalgorithm until the network learns to associate between the input (thestructure of a track) and the appropriate output (the energy spectrum).

Several important features of neural architectures distinguish them fromprior art approaches.

1. There is little or no executive function. There are only very simpleunits each performing its sum of products calculation. Each processingelement's task is thus limited to receiving the inputs from itsneighbors and, as a function of these inputs, computing an output valuewhich it sends to its neighbors. Each processing element performs thiscalculation periodically, in parallel with, but not synchronized to, theactivities of any of its neighbors.

2. All knowledge is in the connections. Only very short term storage canoccur in the states of the processing elements. All long term storage isrepresented by the values of the connection strengths or "weights"between the processing elements. It is the rules that establish theseweights and modify them for learning that primarily distinguish oneneural network model from another. All knowledge is thus implicitlyrepresented in the strengths of the connection weights rather thanexplicitly represented in the states of the processing elements.

3. In contrast to algorithmic computers and expert systems, the goal ofneural net learning is not the formulation of an algorithm or a set ofexplicit rules. During learning, a neural network self-organizes toestablish the global set of weights which will result in its output fora given input most closely corresponding to what it is told is thecorrect output for that input. It is this adaptive acquisition ofconnection strengths that allows a neural network to behave as if itknew the rules. Conventional computers excel in applications where theknowledge can be readily represented in an explicit algorithm or anexplicit and complete set of rules. Where this is not the case,conventional computers encounter great difficulty. While conventionalcomputers can execute an algorithm much more rapidly than any human,they are challenged to match human performance in non-algorithmic taskssuch as pattern recognition, nearest neighbor classification, andarriving at the optimum solution when faced with multiple simultaneousconstraints. If N exemplar patterns are to be searched in order toclassify an unknown input pattern, an algorithmic system can accomplishthis task in approximately order N time. In a neural network, all of thecandidate signatures are simultaneously represented by the global set ofconnection weights of the entire system. A neural network thusautomatically arrives at the nearest neighbor to the ambiguous input inorder 1 time as opposed to order N time.

Training of the neural network may be done in various ways including, inparticular, the back-propagation technique, which is described in Clark,J. W., "Neural network modeling", Phys. Med. Biol., 36, 1259-1317(1991), and Rumelhart, David E. and McClelland, James L., "ParallelDistributed Processing", MIT Press, 1986, Volume 1, both of which arehereby incorporated herein by reference. During net training, errors(i.e., the difference between the appropriate output for an exemplarinput and the current net output for that output) are propagatedbackwards from the output layer to the middle layer ,(or layers) andthen to the input layer. These errors are utilized at each layer by thetraining algorithm to readjust the interconnection weights so that afuture presentation of the exemplar pattern will result in theappropriate output category. The back-propagation learning algorithm isbased on least squares minimization of the network error defined as thedifference between the actual output and the desired output.

In the present application of a neural network the input of the trainingpairs are the parameters describing the structures of the particletracks and the output is the energy spectra. Those parameters mayinclude, in particular: type of recoil particle, track length and trackradius. Other track parameters associated with the details of themicrodosimetric dose distribution within the track may be defined andinputted.

The input/output training sets as well as test data are generated usinga Monte Carlo simulation technique. A three-dimensional realisticsimulation package is developed to obtain detailed computer simulationof how high LET particles are transported through the ORAM material. Thesimulation package will be based on the formalism developed by Zaider etal., which is described in Zaider M., Benner D. J. and Wilson W. E.,"The Application of Track Calculations to Radiobiology. I. Monte carloSimulation of Proton Tracts", Rad. Res. 95, 231-247 (1983), herebyincorporated herein by reference. The Monte Carlo simulation isperformed in three steps: (1) generation of the tracks, includinggeometrical coordinates of interaction points, energy deposition andtype of event; (2) generation of primary electrons resulting frominteraction of the high LET particles with the medium; and (3) totransport electrons in the medium to simulate the radial dosedistribution in the particle track. The calculations can be applied tothe variety of heavy ions types and energies. The Monte Carlo resultsare used to develop the energy discrimination algorithm by training theneural network.

The neural network and the Monte Carlo simulation are implemented on,for example, a SUN workstation or other suitable computing device,including commercially available neurocomputer accelerator boards.

Dose distributions can be calculated for several radiation types andenergies using published data, including 0.3-20 MeV protons, 930 MeV ⁴He and 41 MeV ¹⁶ O ions. See Wingate C. L. and Baum, J. W., "MeasuredRadial Distribution of Dose and LET for Alpha and Proton Beams inHydrogen and Tissue-Equivalent Gas", Rad. Res., 65 1-19 (1976), Varma M.N., Baum J. W., and Kuehner A. V., "Radial Dose, LET, and W for ¹⁶ OIons in N₂ and Tissue-Equivalent Gases", Rad. Res. 70, 511-518 (1977);Wilson W. E., Metting N. F. and Paretzke H. G., "Microdosimetric Aspectsof 0.3- to 20 MeV Proton Tracks", Rad. Res. 115, 389-402 (1988), andVarma M. N. Paretzke H. G., Baum J. W., Lyman J. T., and Howard J.,"Dose as a Function of Radial Distance From a 930 MeV ⁴ He Ion Beam", InProc. 5th Symp. Microdosimetry, Sep. 22-26, 1975 (EUR 5452 d-e-f-). Thetrack, in first approximation can be described as a cylindrical-shapedregion of excitations and ionizations. The radial dose distributionwithin the track can be calculated based on published experimental datain tissue equivalent gas. To calculate the simulated radial distance(r') and the radial dose distribution D(r') in the ORAM material, thefollowing transformation can be applied (see Varma et al. supra):

    r'=r{[(S/ρ)ρ].sub.gas /[(S/ρ)ρ].sub.ORAM } (1)

and

    D(r')=D.sub.gas (r){[(Z/A)(S/ρ)].sub.ORAM /[(Z/A)(S/ρ)].sub.gas }(ρ.sub.ORAM /ρ.sub.gas).sup.2                    (2)

where S/ρ is the mass stopping power, ρ is the density and Z and A arethe atomic and mass number of the target material, respectively.

Next, the "bit-flip" probability in response to high LET particles canbe calculated. The particle interacting with the ORAM material causesthe local temperature to increase, as a result of energy dissipation.Preliminary results have shown temperature increase of up to 95° C.inside 1 MeV proton tracks. Since it is known that ORAM are temperaturesensitive, this temperature increase is expected to change theinformation originally written (cause "bit-flips"). The amount andspatial distribution of those bit-flips can be calculated, as follows:(1) calculation of the temperature distribution within the particletracks based on the calculated dose distribution, and (2) theoreticalanalytical calculation of the distribution of "bit-flips" as a functionof energy, temperature and time, it can be assumed that the number of"bit-flips" follows first order kinetics, i.e.:

    N(t)=N.sub.o (1-e.sup.-kt)                                 (3)

where N(t) is the number of "bit-flips" at the time t, N_(o) is theinitial concentration of bits in the "read" form, and k is the rateconstant. To consider the effect of temperature on the rate of bittransformation, the Arrhenus rate equation is used: ##EQU1## where A isa factor which is only slightly temperature dependent and will beassumed to be constant, and E is the difference in the energy betweenthe two molecular states. Calculations based on track structure theory(TST) to predict the response of 3-D ORAM to proton radiation in theenergy range 1-3 MeV have been made. The results show that (1) theprobability of proton induced bit-flips is up to three orders ofmagnitude higher than the probability of spontaneous bit-flips at roomtemperature, and (2) the probability of room temperature spontaneousbit-flips (i.e. not induced by radiation) increases as a function oftime since the bits were written.

The 3-D ORAM material that was studied, is composed of apolymethylmethacrylate (PPMA) matrix, that contains 1% by weightspirobenzopyran as photochromic molecule. The spiropyran molecule can beexcited into merocyanine form by the simultaneous absorption of either a1064 nm photon and 532 nm photon, or two 532 nm photons. The merocyaninemolecule is exited by absorption of two 1064 nm photons followed byde-excitation and emission of red-shifted fluorescence, returning to thespiropyran form. Equations (3) and (4) were applied, using theparameters given by Parthenopoulos et al., supra, to calculate the rateof conversion of the merocyanine form to spiropyran form. The values ofthe parameters A_(i) and E_(i) in Equation (4) were calculated based ondata given by Gardlund et al. Gardlund Z. G., and Laverty J. J.,"Polyalkylmethacrylate Films as Matrices for Photochromic Studies",Journal of Polymer Science, Polymer Letters 7 719 (1969). The change ofthe photochromic molecule (spirobenzopyran) between its two isomericforms records a bit-flip. The interaction of ionizing radiation with thematerial will cause bit-flips by causing the transformation of themerocyanine form of the molecule back to its spiropyran form. The radialdistributions of the bit-flip probability along the tracks of 1 and 3MeV protons were calculated and compared to the background bit-flipprobability (bit-flips that are not induced by radiation). Thetheoretical calculations were applied to the experimental results ofWingate et al., supra, who measured the radial dose distribution for 1and 3 MeV protons in tissue-equivalent gas. The radial temperatureincrease along the proton track was calculated using the heat capacityof the material, and the radial dose distribution.

The probability of radiation induced bit-flip was compared to the noiseof the system, i.e., the probability that a bit-flip will occurspontaneously (not induced by radiation). The results clearlydemonstrate that significant radiation induced bit transformationprobability is predicted. The radiation induced bit-flip probability canbe by three orders of magnitude higher than the probability ofspontaneous bit-flip. These probabilities are expected to increase withthe atomic number Z of the incident particle, even for much higherenergies typical to space radiation environments.

It is expected therefore that a high LET dosimeter based on SP will besensitive to low dose levels. A potential problem was identified wherethe probability of room temperature spontaneous bit-flips (i.e. notinduced by radiation) increases as a function of time since the bitswere recorded (fading). The room temperature fading seems to be alimitation of the SP material. It is not expected however that thislimitation will prevent this material from being used as a high LETdosimeter. A ORAM dosimeter can in principle be thermoelectricallycooled and used as passive integrating dosimeter even at elevatedtemperature.

Other types of photochromic materials may be selected, as in an effortto reduce the fading effects and eliminate any need tothermoelectrically cool the material. Other materials includephotochromic fulgide, Parthenopoulos, D. A. and Rentzepis P. M.,"Two-Photon Volume Information Storage in Doped Polymer Systems", J.Appl. Phys. 68, 5814-5818 (1990), and spyropyran and other moleculesconsidered as candidates for optical 3D storage memory, such as thosedescribed in Malkin J., Dvornikov A. S., Straub K. D., and Rentzepis P.M., "Photochemistry of Molecular Systems for Optical 3D Storage Memory",Res. Chem. in. 19, 159-189 (1993), which paper is hereby incorporatedherein by reference.

Neutron Spectrometer

Referring now to FIG. 8, a spectrometer for measuring incidentradiation, such as neutron radiation, is designated generally byreference numeral 40. The spectrometer 40 comprises an optical memoryelement 41, a laser assembly 42, a photodetector assembly 43 (including,for example, a photodiode array, a CCD device, or the like), a processor44 and associated computing elements such as ROM and RAM, an inputdevice 45, a display 46, and a communications port 47. Although notshown, the spectrometer includes other conventional devices including apower supply such as a battery with an optional connection to AC powerif desired, and a housing represented by broken lines 49 for containingthe various components of the spectrometer preferably in a compact,lightweight and easily carried package. The optical memory elementpreferably is disposed in the housing so as to be responsive to theradiation to be measured by the spectrometer. An erasing/resettingdevice, such as a heating element, may also be provided for bulkerasure.

The optical memory element 41, laser assembly 42 and photodetectorassembly 43 function generally as described in the above referencedHunter et al. paper which describes a system for writing and readingORAM. The read/write functions of the laser and photodetector assembliesare controlled by the processor 44 that may be a conventionalprogrammable microprocessor of any suitable type. The processorpreferably operates under a program that reads the optical memoryelement preferably repetitively at a desired frequency to obtain realtime measurement of the radiation field being monitored. The threedimensional data is processed by the processor, as through use of aneural net, to obtain the energy spectrum (intensity v. energy),absorbed dose and/or energy. Dose equivalent may be calculated from theabsorbed dose and energy as above discussed, and the dose equivalent,absorbed dose and/or energy spectrum may be displayed on the display 46.The processor also controls resetting of the optical memory element asupon initiation of a new scanning operation, which may be effected bypushing a start button of the input device 45. The display may beperiodically or continuously updated as radiation is accumulated. Thedose rate and dose equivalent may also be provided. In this mannermonitoring of neutron (or other) radiation may be obtained.

The spectrometer may be provided with one or more memory storage devicesfor storing the measured radiation data. The stored data may bedownloaded to other devices as desired for further processing byconventional devices such as via communications port 47. Other outputdevices may be provided as desired such as a printer port for directprinting of data or other information to a printer. The input device 45,for example, a keypad or keyboard, is provided to provide ahuman/processor interface for initiating commands, inputting data,responding to inquiries posed by the processor, etc.

Real-Time Electronic Dosimeter

Referring now to FIG. 9, a real-time dosimeter for measuring incidentradiation, such as neutron radiation, is designated generally byreference numeral 50. The dosimeter 50 comprises an optical memoryelement 51, a laser assembly 52, a photodetector assembly 53, aprocessor 54 and associated computing elements such as ROM and RAM, aninput device 55, a display 56, and a communications port 57. Althoughnot shown, the dosimeter includes other conventional devices including apower supply such as a battery and a case represented by broken lines 49for containing the various components of the dosimeter. The dosimeterpreferably is of small size and weight on the order of a common pagerthat may be easily carried in a pocket, worn on a belt, carried on achain around one's neck, etc. The optical memory element preferably isdisposed in the casing so as to be responsive to the radiation to bemonitored.

The laser assembly 52 necessarily must be a miniaturized assembly using,for example, diode lasers of the type commonly used in compact discplayers, or the like. However, the optical memory element 51, laserassembly 52 and photodetector assembly 53 would still function generallyas described in the above referenced Hunter et al. paper which describesa system for writing and reading ORAM. The read/write functions of thelaser and photodetector assemblies are controlled by the processor 54that may be a conventional programmable microprocessor of any suitabletype. The processor preferably operates under a program thatrepetitively reads the optical memory element at a desired frequency toobtain real time measurement of the radiation field being monitored. Thethree dimensional data is processed by the processor, as through use ofa neural net, to obtain absorbed dose and energy. Dose equivalent may becalculated from the absorbed dose and energy as above discussed, and thecalculated dose equivalent, absorbed dose, dose rate and/or energy (suchas average energy) may be displayed on the display 56. The processoralso controls periodic refreshing of the optical memory element, whetherafter each read or otherwise. In this manner real time monitoring ofneutron (or other) radiation may be obtained.

The dosimeter may be provided with one or more memory storage devicesfor storing the measured radiation data. The stored data may bedownloaded to other devices as desired for further processing byconventional devices such as via communications port 57. Other outputdevices may be provided as desired such as a printer port for directprinting of data or other information to a printer. The input device 55,which may be a keypad, is provided to provide a human/processorinterface for initiating commands, inputting data, responding toinquiries posed by the processor, etc.

Although the invention has been shown and described with respect tocertain preferred embodiments, equivalent alterations and modificationswill no doubt occur to others skilled in the art upon the reading andunderstanding of this specification. Moreover, while a particularfeature of the invention has been described with respect to only one orless than all of the illustrated embodiments, such feature may becombined with one or more features of the other embodiments, as may bedesired and advantageous for any given or particular application. Thepresent invention includes all such equivalent alterations andmodifications, and is limited only by the scope of the following claims.

What is claimed is:
 1. A high LET radiation dosimetry method comprisingthe steps ofstoring information in a three dimensional optical memoryelement having a plurality of memory locations by exciting the memorylocations from a first energy state to a second energy state, exposingthe optical memory element to high LET radiation to alter theinformation stored in the optical memory element as a function of theradiation to which the optical memory element is exposed, the excitedmemory locations reverting from their second energy state to their firstenergy state through localized interactions between the radiation andmolecules composing the optical memory element, retrieving the alteredinformation from the optical memory element for subsequent analysis byreading the memory locations to determine the spatial distribution ofthe memory locations that have reverted to their first energy state,analyzing the altered information retrieved from the optical memoryelement to extract radiation dose information therefrom, said analyzingstep including using a neural network computer apparatus for determiningthe radiation energy as a function of the spatial distribution of thememory locations that have reverted to their first energy state.
 2. Amethod as set forth in claim 1, wherein the optical memory elementcomprises a volume of a transparent polymer doped with a light sensitivechemical dopant molecule.
 3. A method as set forth in claim 2, whereinthe light sensitive chemical dopant is spirobenzopyran.
 4. A method asset forty in claim 1, wherein said optical memory device issubstantially composed of hydrogen and carbon.
 5. A method as set forthin claim 1, wherein the retrieving step includes reading the memorylocations to determine the number of the memory locations that havereverted to their first energy state, and the analyzing step includesdetermining the radiation dose as a function of the number of the memorylocations that have reverted to their first energy state.
 6. A method asset forth in claim 1, wherein the retrieving step includes usingtwo-photon absorption to read the memory locations.
 7. A method as setforth in claim 1, wherein the storing step includes using two-photonabsorption to write the memory locations by exciting them to theirsecond energy state.
 8. A method as set forth in claim 1, wherein saidhigh LET radiation is neutron radiation.
 9. A method as set forth inclaim 1, wherein said high LET radiation is proton radiation.
 10. Anapparatus comprising a three dimensional optical memory element having aplurality of memory locations that may be written from a first energystate to a second energy state, and which memory locations becomealtered by reversion from their second energy state to their firstenergy state through localized interactions between incident radiationand molecules composing the optical memory element, means for readingthe optical memory element to retrieve therefrom information alteredthrough interaction with incident radiation, means for analyzing thealtered information retrieved from the optical memory element to extractradiation dose information therefrom, and means for displaying the doseinformation extracted from the optical memory element.
 11. A apparatusas set forth in claim 10 wherein said means for analyzing includes aneural network computer apparatus for determining the radiation energyas a function of the spatial distribution of the memory locations thathave reverted to their first energy state.
 12. An apparatus as set forthin claim 10, wherein said means for analyzing includes means fordetermining both radiation dose and energy.
 13. An apparatus as setforth in claim 10, wherein the optical memory element comprises a volumeof a transparent polymer doped with a light sensitive chemical dopantmolecule.
 14. An apparatus as set forth in claim 13, wherein the lightsensitive chemical dopant is spirobenzopyran.
 15. An apparatus as setforth in claim 10, wherein said optical memory device is substantiallycomposed of hydrogen and carbon.
 16. A real time dosimeter comprisingthe apparatus of claim 10, wherein the dose information is displayed ona real-time basis.
 17. A spectrometer comprising the apparatus of claim10, wherein radiation dose and energy are displayed on a real-timebasis.